Oscilloscope



Sept. 25, 1962 F. L. KATZMANN OSCILLOSCOPE Filed June 11, 1959 R m r l m F b C 2 2 lllll l lllllllllllllll I- MQSHZMHOQ m4 SFZMFOQ QZCOM EMQ $25309 FRED L.KATZMANN BY 291w 8 BEAM POSITION Fig. 2

ATTORNEYS United States Patent 3,056,063 OSCILLOSCOPE Fred L. Katzmann, Cedar Grove, N.J., assignor, by mesne assignments, to Fairchild Camera and Instrument Corporation, Syosset, N.Y., a corporation of Delaware Filed June 11, 1959, Ser. No. 819,769 13 Claims. (Cl. 315-22) This invention relates to an oscilloscope, and more particularly to focussing the light spot thereof.

An oscilloscope converts electrical signals into a visual display, the display proper being produced by a beam of electrons that impinges upon a fluorescent screen. At the point of impingement, a glowing spot of light is produced; and when the electron beam is deflected, the point of impingement moves thus producing a glowing line that traces out the display. To obtain fine detail, the glowing spot should be as small as possible. Focussing the electron beam, in a manner similar to the focussing of light rays, causes the point of impingement to be minimal. Unfortunately, for reasons which need not be discussed here, the electron beam tends to be defocussed as it moves away from the center of the display.

The problem of spot defocussing occurs in most cathode ray tube apparatus. In oscilloscopes theproblem is complicated by the fact that the spot may move at various speeds. This precludes the use of tuned circuits that solve the problem in other cathode ray tube apparatus.

It is therefore the principal object of my invention to provide an improved focussing circuit for a cathode ray tube.

It is another object of my invention to provide a circuit that maintains a focussed light spot, regardless of its position on the display.

The attainment of these objects and others will be realized from the following specification, taken in conjunction with the drawings, in which,

FIG. 1 is a schematic representation of my invention; and

FIG. 2 depicts associated waveforms.

My invention contemplates the production of a focussing voltage whose value varies 'with the position of the electron beam.

To facilitate the explanation of my invention, the following terms will be used. Upvolting means raising the potential without implying that the potential is positive, negative, or necessarily under-goes any change in sign. Similarly, downvolting means lowering the potential.

My invention may be understood from FIG. 1. Let us first consider the condition when the light spot is at the center of the display. At this time deflection plates 16 and 18 are at exactly the same potential, and therefore do not affect the transiting electrons, this condition existing when deflecting waveforms 24 and 26 are at their midpoints A and B. Electrons emitted from cathode 12 traverse focus electrode 14, and pass unafiectedly between the deflection plates. 'FIG. 2a shows the relation between the deflecting potentials and the position of the electron beam.

In FIG. 1, coupling-decoupling diodes 28 and 30 have their anodes 32 and 34 connected to deflection plates 16 and 18, respectively. When the electron beam is centered, their anodes are therefore at the same potential. Potentiometer 44 is now adjusted so that junction 40 and cathodes 36 and 38, which are connected to junction 40, have a potential that is low enough to just cause diodes 28 and 30 to conduct.

Let us now consider the dynamic condition, i.e., when the light spot is moving. To achieve this result, deflection tubes 20 and 22 are energized. The positivegoing portion B-C of waveform 26 attracts the electron beam to plate 18, while the negative-going portion AD of waveform 24 repels the electron beam from plate 16. The coaction of these potentials deflects the electron beam as shown by the dotted electron beam path of FIG. 1.

According to my invention, the negative-going potential of waveform 24 downvolts anode 32 of diode 28 to a potential lower than cathode 36. Diode 28 thereupon becomes nonconductive, and decouples junction 40 from deflection plate 16. At diode 30 the situation is different. Here, the positive-going waveform 26 upvolts anode 34 of diode 30. This diode remains conductive, and couples the waveform of the deflection system to junction 40. As a result, junction 40 is upvolted in accordance with deflection plate 18.

As deflection plate 18 is upvolted to progressively higher values, it produces two simultaneous effects. Firstly, acting through conductive diode 34, it increases the potential at point 40; and secondly, it helps deflect the electron beam to the right as indicated by the dotted line. Thus, there is a relation between the potential at point 40 and the position of the electron beam. This is shown in FIG. 2b.

A similar result is achieved for electron beam positions to the left of the displays center. In this case, the operation of the diodes is reversed. That is, the negativegoing portion BE of waveform 26 repels the electron beam, and downvolts diode 30 to cutoff, whereupon it decouples junction 40 from deflecting plate 18. Simultaneously, the positive-going portion AF of waveform 24 attracts the electron beam, causes diode 28 to remain conductive, and upvolts junction 40.

It may therefore be seen from FIG. 2b that when the electron beam is at the center of the faceplate, point 40 is at a potential established by potentiometer 44. As the electron beam is deflected to either side of the faceplate, the potential at point 40 increases progressively with the displacement of the electron beam.

Since the potential at point 40 varies with the position of the light spot, I couple it to focussing electrode 14, as will be described hereinafter. In this way, the focussing potential is varied in accordance with the position of the light spot.

While many cathode ray tubes exhibit the aforementioned dynamic defocussing characteristic, many of them are satisfactorily focussed for a short distance on either side of the tubes center. Ideally then, the focussing potential should be constant over a given central range, and should then vary with increased deflection.

My circuit permits a satisfactory solution for this condition. To do this, potentiometer 44 is adjusted so that point 40 is at a higher potential than previously. At this setting, both diodes are non-conductive, and therefore the potential at junction 40 remains constant at the value established by potentiometer 44. Now, the positive-going deflection waveform must be upvolted a given amount which means that the electron beam must move an appreciable distance before one of the diodes becomes conductive. Only then does the potential at point 40 begin to increase as previously described. Under these conditions, the focussing potential takes the shape shown in FIG. 2c.

If, for some applications, it is desired that the focussing potential decreases with larger deflections, diodes 28 and 30 are reversed.

It is known that there is an optimum potential relationship between the cathode of the cathode ray tube and the focussing electrode. Since cathode 12 is generally at a very low potential, it is desirable that the focussing electrode 14 be at a potential fairly close thereto rather than at the potential at point 40. To achieve this result, I connect point 40 to a voltage divider comprising resistances 46, 48 and potentiometer 50 whose slider 51 s connected to focussing electrode 14. In this way, ocussing electrode 14 may have applied thereto a potenial which is a desired amount below that at point 40, nd above the potential at cathode 12.

Unfortunately, the voltage divider also causes slider "1 to have a smaller voltage swing than point 40. I overome this shortcoming by use of bootstrapping tube 52. his is connected across a portion of the voltage divider that all intermediate points thereof have the same otential swing as junction 40. In this way (1) focus lectrode 14 has the desired potential relation to the poantial at cathode 12, and (2) the full range of potential will Die to the bootstrapping effect of tube 52, the voltage ivider and associated wiring is upvolted and downvolted 1 correspondence with point 40. The stray capacitance ssociated with these elements may introduce a deleterious [feet upon the waveform of the focus voltage. To ovcrome this, I use capacitance 54 whose value is so large Jmpared to that of the stray capacitance that the effect E the stray capacitance is minimized.

The above circuit may of course be used to correct efocussing due to horizontal deflection and/or vertical :fiection.

Cathode ray tubes are also subject to a spot shape dislrtion known as astigmatism, which also tends to vary ith spot position. An astigmatism correction electrode therefore usually part of the tube structure. In the 1st, a manually adjustable potential was applied thereto.

[y invention, as disclosed above, may be used to vary the )tential applied to the astigmatism correction electrode, 1d thus correct this type of distortion. It will therefore understood that the term spot shape correcting elecade refers to both focus and astigmatism correcting ectrodes, and that spot shape correcting potential cludes both focus potentials and astigmatism correcting tentials.

What is claimed is:

l. A combination for correcting the spot shape distorm due to depositioning an electron beam in a cathode y tube, comprising: a cathode ray tube; means for rming an electron beam in said tube, said means comising an electron emitting cathode; means for decting said electron beam, said deflecting means comising a pair of deflecting plates positioned astride the th of said electron beam; a spot shape correcting elec- K16 within said tube; a junction; means applying se- :ted potentials from said deflection plates to said juncn, said means comprising a first diode having its anode nnected to one of said plates, a second diode having anode connected to said other deflection plate, and

:onnection between the cathodes of said diodes and said lction; means for preventing for a predetermined interthe application of said selected potentials to said ltes, said means comprising: a potential source, a poitiometer connected across said potential source and a unection between the slider of said potentiometer and d junction; means to apply said selected potentials to d electrode at a D.C. level diflerent than that at said lction, said means comprising a voltage divider having e end thereof connected to said junction, said voltage 'ider comprising a first resistance, a second potentiomeand a second resistance, and connection between the ler of said second potentiometer and the electrode; ans to apply said selected potentials in their full amtude, said means comprising a voltage regulator tube nnected between said junction and the junction of said 0nd potentiometer and said third resistance; means bias said cathode relative to said electrode, said means nprising a connection between said cathode and a point said voltage divider; and means to minimize the effect itray capacitance, said means comprising a capacitance lnected across said first resistance and said second potiometer of said voltage divider.

I. A combination for correcting the spot shape distor- 1 due to depositioning an electron beam-in a cathode ray tube, comprising: a cathode ray tube; means for forming an electron beam in said tube, said means comprising an electron emitting cathode; means for deflecting said electron beam, said deflecting means comprising a pair of deflecting plates positioned astride the path of said electron beam; a spot shape correcting electrode within said tube; a junction; means applying selected potentials from said deflection plates to said junction, said means comprising a first diode having a first electrode connected to one of said plates, a second diode having a corresponding electrode connected to said other deflection plate, and a connection between the other electrodes of said diodes and said junction; means for preventing the application of said selected potentials for a predetermined interval, said means comprising: a potential source, a potentiometer connected across said potential source and a connection between the slider of said potentiometer and said junction; means to apply said selected potentials to said electrode at a D.C. level different than that at said junction, said means comprising a voltage divider having one end thereof connected to said junction, said voltage divider comprising a first resistance, a second potentiometer and a third resistance, and a connection between the slider of said second potentiometer and said electrode; means to :pply said selected potentials in their full amplitude, said means comprising a voltage regulator tube connected between said junction and the junction of said second potentiometer and said third resistance; and means to bias said cathode relative to said electrode, said means comprising a connection between said cathode and a point on said voltage divider.

3. The combination of claim 2 wherein the cathodes of said diodes are connected to said junction.

4. The combination of claim 2 wherein the anodes of said diodes ar connected to said junction.

5. The combination comprising: a cathode ray tube; electron beam forming means positioned within said tube; a pair of electron beam deflecting plates positioned astride the path of said electron beam; a spot shape correcting electrode within said tube; a first diode having a first electrode connected to one of said plates; a second diode having a corresponding electrode connected to said other deflection plate; a connection between the other electrodes of said diodes and a junction; a potential source; a potentiometer connected across said potential source; a connection between the slider of said potentiometer and said junction; a voltage divider having one end thereof connected to said junction, said voltage divider comprising a second potentiometer; a connection between the slider of said second potentiometer and said electrode; a voltage regulator tube connected across said voltage divider; a connection between said electron beam forming means and a point on said voltage divider.

6. In combination with a cathode ray tube wherein the shape of the light spot is distorted by being depositioned, said tube having an electron emitting cathode, a pair of electron beam deflecting plates, and a spot shape correcting electrode, a circuit for correcting spot shape distortion due to spot depositioning, comprising: a pair of coupling-decoupling diodes having similar electrodes thereof connected to respective said deflection plates and having their other electrode connected to a junction; a potential source; a potentiometer connected across said potential source; a connection between the slider of said potentiometer and said junction whereby the coupling-decoupling state of said diodes may be determined; a voltage divider having one end thereof connected to said junction, said voltage divider comprising a second potentiometer; a connection between the slider of said second potentiometer and said electrode whereby a spot shape correcting potential may be applied; and a connection between a point on said voltage divider and said cathode whereby a suitable bias may be applied to said cathode.

7. In combination with a cathode ray tube wherein the shape of the light spot is distorted by being depositioned from a center position, said tubing having a pair of electron beam deflecting plates, and a spot shape correcting electrode, circuit means for correcting spot shape distortion comprising: a pair of coupling-decoupling diodes having corresponding electrodes thereof connected to respective said deflection plates and having their other electrode connected to a junction; means connecting said junction to said correcting electrode to provide a potential at said last electrode proportional to deflection of said spot from said center position; a potential source; a potentiometer connected across said potential source; and a connection between the-slider of said potentiometer and said junction.

8. The combination comprising: a deflection system for a cathode ray tube; a spot shape distortion correcting electrode in said tube, said distortion being proportional to distance of said spot from a reference position; a first coupling-decoupling diode connected between said electrode and a point of said deflection system; a second coupling-decoupling diode connected between said electrode and another point of said deflection system; a common junction connecting together an electrode of each said diode; means connected between said junction and said correcting electrode to apply a correction potential to said correcting electrode proportional to said distance from said reference position; and means applying a bias potential to said diodes to cause both said diodes to remain non-conductive for a given interval.

9. In combination with a cathode ray tube having electron beam forming means, electron beam deflecting means comprising deflection potentials whereby deflecting said beam produces a glowing spot whose shape changes with the position of said spot, and a spot shape control electrode, a spot shape correcting circuit comprising: means applying a portion of said deflection potential to said electrode in proportion to said position to control said spot shape, said means comprising a first coupling-decoupling device connected to one point of said deflecting means, and a second coupling-decoupling device connected to another point of said deflecting means, a connection between said devices, means connecting said last connection and said electrode.

10. In combination with a deflection system for producing deflection signals for the electron beam of a cathode ray tube wherein the glowing spot thereof has its shape distorted according to the spots position, the improvement comprising: a spot shape distortion correcting electrode positioned in said tube; means connected to said deflection system for deriving a portion of said deflection signals for use as a spot shape correcting potential whose instantaneous value depends upon the spots position, said means comprising a coupling-decoupling device; and means applying said spot shape correcting potential to said electrode, said means comprising a potentiometer connected to said device and a connection between the slider of said potentiometer and said electrode.

11. In combination with a deflection system for producing deflection potentials for the electron beam of a cathode ray tube wherein the glowing spot thereof has its shape distorted according to the spots position, the improvement comprising: a spot shape distortion correcting electrode positioned in said tube; means connected to said deflection system, for deriving a portion of said deflection potentials for use as a spot shape correcting potential whose instantaneous value depends upon the spots position, said means comprising a coupling-decoupling device; and means applying said spot shape correcting potential to said electrode, said means comprising a connection between said device and said electrode.

12. The combination comprising a cathode ray tube, a deflection system for producing deflection potentials for the electron beam of said tube, the glowing spot produced by the impingement of said beam on the face of said tube being distorted in accordance with the position of said spot with respect to a reference position; a spot shape distortion correcting means for said tube, and means for applying said deflection potentials to said distortion correcting means whereby said distortion correcting means receives a potential having an instantaneous value corresponding to the position of said spot.

13. The combination comprising a cathode ray tube, a deflection system for'producing deflection potentials for the electron beam of said tube, the glowing spot produced by the impingement of said beam on the face of said tube being distorted in accordance with the posi- .tion of said spot with respect to a reference position; a spot shape distortion correcting means for said tube, means for applying said deflection potentials to said distortion correcting means whereby said distortion correcting means receives a potential having an instantaneous value corresponding to the position of said spot, and means for preventing the application of said potential to said distortion correcting means until the instantaneous value exceeds a predetermined level which corresponds to a certain position of said' spot on the face of said tube.

References Cited in the file of this patent UNITED STATES PATENTS 

